Recently, new technology has rapidly developed to achieve a clean energy society. Great efforts have been made to achieve earth-friendly society. For example, efforts have been made which aim to achieve a petroleum-free society and zero emission, and spread of power saving products. Particularly, batteries have been recently in the spotlight such as a storage battery which is capable of storing a large quantity of electricity to transfer energy to and from an electric vehicle or to transfer energy during the occurrence of disaster and emergency, and a secondary battery that is used in a portable electronic apparatus. Examples of such batteries that have been known include a lead storage battery, an alkali storage battery, a lithium ion secondary battery and the like. Particularly, the lithium ion secondary battery is capable of realizing high capacity and a reduction in size and weight. Furthermore, the lithium ion secondary battery has excellent characteristics such as high output and a high energy density, and thus the lithium ion secondary battery has been commercialized as a high-output power supply of an electric vehicle, an electromotive tool and the like. Further, development of a material for a next generation lithium ion secondary battery is actively in progress in the world. In addition, as an example of collaboration with a house and a battery, a home energy management system (HEMS) is known. Further, attention has been gained to a system that can wisely consume energy through management of automatic control, optimization of supply and demand of electric power and the like, and such a management is performed by integrating a control system and information relating to home electricity such as the smart home appliance, the electric vehicle, solar cell power generation and the like.
Typical examples of a positive electrode active material for the lithium ion secondary battery, which has been put into practical use under these circumstances, include LiCoO2 and LiMnO2. However, Co is unevenly distributed on the earth, and is a rare resource. Accordingly, there are concerns that it is difficult to stably supply Co and the product cost rises when a large amount of Co is used. Therefore, research and development of a positive electrode active material, which can be used instead of LiCoO2, have been actively in progress such as spinel-type LiMn2O4, ternary LiNi1/3Mn1/3CO1/3O2, lithium iron oxide (LiFeO2) and lithium iron phosphate (LiFePO4) as a positive electrode active material.
Among these positive electrode active materials, LiFePO4, which has an olivine structure, has attracted attention, as LiFePO4 is inexpensive, high in stability and rich in deposits.
The olivine-type positive electrode active material, which is represented by LiFePO4, contains phosphorus as a constituent element, and thus phosphorous and oxygen are connected by a strong covalent bond. Accordingly, LiFePO4 is a material which is excellent in stability. The reason is that oxygen is not released from LiFePO4 at a high temperature unlike a positive electrode active material such as LiCoO2, and there is no firing risk since oxidative decomposition of an electrolytic solution does not occur.
However, even in LiFePO4 having the above-described advantages, there is a problem with low electron conductivity. The cause of the problem is considered such that diffusion speed of lithium ions is slow at the inside of LiFePO4 and therefore electron conductivity is low, and such a diffusion speed is originated from a molecular structure thereof.
As a positive electrode material in which the electron conductivity is improved, for example, a positive electrode material is disclosed in which a plurality of primary particles of the positive electrode active material composed of LixAyBzPO4 (provided that, A represents at least one selected from the group consisting of Co, Mn, Ni, Fe, Cu and Cr, B represents at least one selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, Y, and a rare-earth element, and relationships of 0<x≦2, 0<y≦1, and 0≦z≦1.5 are satisfied) are aggregated to form secondary particles, carbon as an electron conductive material is interposed between the primary particles, and a surface of the positive electrode active material is coated with a carbonaceous film. Further, with regard to the positive electrode material that is manufactured by spraying and drying slurry containing a positive electrode active material or a precursor of the positive electrode active material and an organic compound to prepare a granule, and subjecting the granule to a heat treatment under a non-oxidizing atmosphere at 500° C. to 1000° C., methods are disclosed wherein electron conductivity of a carbonaceous film is improved by increasing a density of the granule to coat the surface of the positive electrode active material with the carbonaceous film having a uniform thickness (for example, refer to Patent documents 1 to 5).
When the aforementioned positive electrode materials which have improved electron conductivity are used for a battery, an improvement in an initial battery capacity is reliably observed. However, deterioration in the battery capacity is also observed when the battery is maintained for a long period of time in a charged state, or with respect to cycle characteristics in which charge and discharge are repetitively carried out. Accordingly, there is a demand for an improvement in durability.
In order to improve the durability, for example, a method is disclosed wherein a compound, which is composed of at least one of sulfur (S), phosphorus (P) and fluorine (F), is adhered to a surface of composite oxide particles (for example, refer to Patent document 6).
However, the method is not preferable as a method of improving the durability, since a gas is generated in the method due to the above-described compound and thus a battery is expanded, or the compound covers a surface of the electrode and thus the electron conductivity is inhibited.
It is considered that the deterioration in the durability is caused by metal impurities which are eluted to an electrolytic solution. The metal impurities, which are eluted to the electrolytic solution, are electrodeposited on a surface of a negative electrode, break a solid electrolyte interface (SEI) film, and then form a SEI film again, thereby deteriorating the battery capacity. In addition, there is a concern that the metal impurities, which are eluted to the electrolytic solution, break through a separator and thus short-circuiting of the battery may be caused. As a method of solving the problems, for example, a manufacturing method is disclosed wherein a magnetic metal compound is removed by using a magnet so as to suppress elution of the metal impurities to the electrolytic solution (for example, refer to Patent document 7).
This method is effective to remove a magnetic metal compound such as iron and nickel. However, in this method, it is difficult to remove a non-magnetic compound such as manganese and impurities which are present in a form of a non-magnetic compound, even if the impurities are metal compounds such as iron and nickel. In addition, such a method requires a complicated manufacturing process, thereby leading to an increase in the cost, and therefore the method is not preferable.